There are several ways to accomplish your goal. We can give you different Ideas, with Pros and Cons.
If you want to have a Map fixed in size, there is nothing against using FromImage.
The Upsides would be that you dont need to tweak your BiomePlacements.
The Downsides would be that you need to place EACH biome. That could make adding tiny biomes a pain.
Thats also possible with normal mode. Its a bit of a pain. But entirely Possible.
As for the Tutorial, i dont think its neither good nor bad. There is not much real value to be gained from this Tutorial.
I would think a tutorial should cover how you actually make a decent looking map.
This comes down to the Quality of your Biomemap. Each Pixel represents 4x4 Blocks. That means, if you have 16 pixels in a straight line, you will probably get a very noticable border.
While TC will randomize your map a little, there can easily be some parts that look artificial.
As for Jupiters approach/idea:
Its not bad, but the problem you mentioned is there. You sadly dont have too much control over the Size of the Islands or Groups.
There would be a ton of tweaking neccessary.
You can NOT replace the default Ocean Biome. You can tweak the Ocean Biome Config to your liking. But Afaik you cannot change which biome he uses as Ocean.
In Jupiters Idea that isn´t a problem though. Aswell, if you use FromImage it isnt a concern either.
As for Tutorials, I saw a From Image Tutorial floating around somewhere:
Here he shows actually how he shapes the Map.
Also, I´ve tried to explain my Approach for the FromImage map here:
On the Follow Up Post, there is a better refined version of my map. This took some effort.